1. Field of the Invention
The present invention relates to an anticorrosion primer coating composition for metals of the type generally called a "wash primer", "shop primer", "etch primer" or "prefabrication primer". More particularly, the invention relates to an anticorrosion primer coating composition for metals which comprises as indispensable components (i) a polyvinyl butyral resin, (ii) a mixture of a borate compound and a molybdate compound or a polyphosphate compound and (iii) phosphoric acid and which may further comprise a phenolic resin and/or a metal powder according to need.
2. Description of the Prior Art
Steel frames and metal sheets or plates for steel structures such as ships and bridges have heretofore been ordinarily coated with anticorrosion primer coating paints, that is, shop primers, after complete removal of rust and mill scales by sand blasting so as to prevent rusting during processing or fabrication.
For example, D. A. Bayliss et al. proposed and examined various prefabrication primers (shop primers) in the Journal of the Oil and Colour Chemists' Association, 51, pages 792-915 (1968) published by the Oil and Colour Chemists' Association.
Shop primers include wash primers for which the conversion treatment is conducted simultaneously with the coating operation, namely shop primers called "shop primers of the conversion treatment type".
A typical composition for such wash primer comprises a polyvinyl butyral resin as a vehicle, phosphoric acid, chromic anhydride, zinc chromate and an alcohol.
The above-mentioned wash primer is produced in two types: a wash primer of the etching type and a wash primer of the high exposure resistance type.
The wash primer of the etching type is suitable for the preliminary treatment of metals to be coated and is prepared by dissolving a polyvinyl butyral resin in an alcohol, kneading and grinding a zinc chromate pigment with the resulting solution and adding an additive containing phosphoric acid to the so prepared main composition. In general, wash primers of this type are used when top coating is effected within a short time from the primer coating and are applied so as to improve adherence between a metal substrate and a top coating layer. In other words, it is not such a strict requirement for single layers of wash primers of the etching type to have high corrosion resistance as it for wash primers of the high exposure resistance type.
As the wash primer of the etching type, there has long been known a composition comprising a polyvinyl butyral resin, phosphoric acid and zinc tetraoxychromate (see the specification of U.S. Pat. No. 2,525,107). There is also known a composition comprising a modified vinyl resin obtained by reacting chromium trioxide and phosphoric acid with a vinyl resin and basic zinc chromate (see the specification of U.S. Pat. No. 2,488,651).
The wash primer of the latter type, namely the wash primer of the high exposure resistance type, comprises a main composition comprising a polyvinyl butyral resin and a phenolic resin as a vehicle and a zinc chromate pigment kneaded and ground with the vehicle and an additive including an alcohol solution of phosphoric acid. In general, the main composition is mixed with the additive at the time of application and the resulting coating composition is applied to form a coating layer excellent in adhesion to a metal substrate and to a top coat paint. Since the wash primer of this type is used when top coating is applied after the lapse of a long time from formation of the primer coating, the wash primer of this type is required to have such a corrosion resistance that the primer coating is resistant to exposure for at least about 6 months.
A typical composition for such wash primer of the high exposure resistance type is disclosed in the specification of U.S. Pat. No. 3,791,850.
In the art, wash primers of the high exposure resistance type have heretofore been mainly used as shop primers.
As will be apparent from the foregoing, conventional wash primers include a zinc chromate pigment or chromic anhydride. Therefore, they entail a problem of chromium pollution. Namely, workers breath in spray dust formed in the coating step and their health is impaired by chromium ions contained in such spray dust.
This chromium pollution is now a serious social problem, and development of an anti-pollution wash primer has been eagerly desired in the art.
For example, attempts have heretofore been made to replace zinc chromate pigments by other anti-pollution pigments. However, coatings formed from wash primers containing such pigments are defective in the corrosion resistance and adhesion. Thus, no practical anti-pollution wash primer has yet been developed in the art.
For example, although an anticorrosion primer paint comprising a polyvinyl butyral resin, zinc chromate and barium metaborate is disclosed in Japanese Patent Public Disclosure No. 44322/73, in this primer paint only a part of the zinc chromate is replaced by barium metaborate and the hygienic problem of prevention of harmful pollution is not completely solved. Further, as is taught in this publicly disclosed specification, a coating from a primer comprising a polyvinyl butyral resin and barium metaborate alone is extremely poor in corrosion resistance and though this wash primer is free of chromium and does not cause a pollution problem, it cannot be used practically.
Further, a paint composition comprising an oil varnish, oil-modified alkyd resin or drying oil as a vehicle and barium borate is known in the art (see the specification of U.S. Pat. No. 2,818,344), but this composition only provides a film having improved mold resistance or chalk resistance and has few properties required of a wash primer.
Still further, the specifications of U.S. Pat. Nos. 3,272,663 and 3,527,860 teach that a molybdate compound is used as an anticorrosion pigment for the wash primer. However, paints of this type are still insufficient in corrosion resistance.
Moreover, an attempt has been made to replace a zinc chromate pigment by a phosphate, for example, zinc phosphate (see the above-mentioned D. A. Bayliss et al. reference). Furthermore, a flame or corrosion resistant paint comprising a white pigment such as silicon phosphate, zirconium phosphate, zinc phosphate or titanium phosphate and an alkyd resin, epoxy resin, air-drying oil, phenolic resin, polyvinyl acetate resin, vinyl chloride resin or polyvinyl acetal resin is known in the art (see the specification of U.S. Pat. No. 3,892,577). These paints, however, have few of the properties required of a wash primer.
As one kind of shop primer, there is known a so-called zinc rich primer formed by incorporating zinc dust at a high concentration in a vehicle. Although this primer has a very high corrosion resistance, it has a fatal defect of poor weldability. Namely, when a steel material coated with this primer is subjected to the welding operation, blow holes are formed by a gas generated from the primer coating and the weld strength is drastically reduced. Furthermore, fumes of zinc generated at the step of welding of the primer-coated material has serious influences on the health of workers and the problems of maintenance of good health conditions is left unsolved.
From a different viewpoint, a so-called electroprint marking process in which an impage is directly formed and developed on a coated steel plate according to the electroprint process has recently been adopted in the ship-building industry so as to save labor.
In this case, a paint excellent in both electric conductivity and corrosion resistance is ordinarily applied to a steel plate, and a wash primer to which electric conductivity has been imparted is especially used for formation of such electrically conductive corrosion-resistant coating. Development of a wash primer free of chromium and capable of eliminating safety and health problems has also been desired in this field where a wash primer is utilized in the electroprint marking process.
As will be apparent from the foregoing, shop primers excellent in corrosion resistance, adhesion to a top coating or over-coating (over-coatability), weldability, adaptability to the electroprint marking process and operation safety (freedom from environmental and hygienic problems at various operation steps) have not yet been developed in the art.
The present invention has now been achieved as a result of our research work made with a view to developing primer coating compositions which overcome the foregoing defects involved in conventional coating compositions.